Method and system for data rate optimization in a digital communication system

ABSTRACT

A method and for optimizing bit rate throughput in a digital communication system is provided. The method includes establishing a relationship between signal to noise ratio and plural symbol rates for a particular constellation size. The method also includes determining noise power spectral density (N(f)), wherein N(f) is determined during a silent period of line probing; determining X k (f), wherein X k (f) is determined by turning on a remote station transmit signal, after N(f) has been measured and determining residual echo E k (f), wherein E k (f) is determined by turning on a central station echo canceller.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to communication systems, and moreparticularly to data rate optimization in a communication system.

[0003] 2. Background

[0004] Various technologies have been developed, such as DigitalSubscriber Line (DSL) technology, to provide high data rates overordinary twisted pair lines. Numerous various standards are currentlyused for transferring data over twisted pair lines.

[0005] One such standard is V.90 (with a later version called V.92)published by the International Telecommunication Union (“ITU”),incorporated herein by reference in its entirety. The V.90/V.92 standarddefines the operation of a digital and analog modem for datatransmission in a digital communication system. The V.90/V.92 standarduses M-ary Pulse Amplitude Modulation (“PAM”) that samples signals atfixed intervals. Typically, a binary input stream is sub-divided intoblock of k bits called symbols and each symbol is represented by a pulseamplitude value with M possible levels where M is the number of pointsin a signal constellation or constellation size.

[0006] In the V.90/V.92 standard, the symbol rate is restricted by G.711PCM codec analog/digital (A/D) sampling rate, which is typically 8 kHz.In order to get a high bit rate (or data rate) a large constellationsize per symbol is required. In PAM systems, every extra bit per symbolrequires more power than in Quadrature Amplitude Modulation (QAM)systems. For example to double the data rate without increasingbandwidth (symbol rate), the constellation size increases from M to M²and energy per bit (E_(b)) will increase from E_(b) toE_(b)=(M²+1)E_(b)/2

[0007] Another standard in the DSL arena is “G.shdsl” or G.992.1,published by ITU and incorporated herein by reference in its entirety.G.shdsl digitally uses Trellis Coded Pulse Amplitude Modulation (TC-PAM)scheme with lower band of frequencies to achieve high performance rangewhile maintaining the ability to symmetrically transmit voice or data.

[0008] In G.shdsl, the constellation size is fixed (16-TCM-PAM) andvariable bit rates are achieved by varying the symbol rate. For a flatto mild amplitude distortion, if symbol rate is unlimited, the bit errorprobability is given by Pe, where$P_{e} = {0.5{Q( \sqrt{\frac{2E_{b}}{N_{0}}} )}}$

[0009] which is independent of data and the consumed bandwidth, andwhere${Q(x)} = {\frac{1}{\sqrt{2\pi}}{\int_{x}^{\infty}{^{{- t^{2}}/2}{t}}}}$

[0010] is the Q-function. However, signal power must increase linearlywith data rates to maintain constant energy per bit (E_(b))

[0011] For channels such as DSL that are distorted or attenuated as afunction of frequency, the trade off between symbol rate and increasingthe number of levels (i.e. increasing the constellation size), must beoptimized in order to fully utilize channel capacity. In a DSLenvironment, cross talk from adjacent pairs in the same bundle increaseswith frequency. Conventional systems (i.e. ITU V.90/V.92 or G.992.1) donot allow the simultaneous choice of optimizing constellation size andsymbol rates to achieve maximum throughput in data rate(s).

[0012] Therefore, there is a need for a system and method for maximizingdata rate by selecting proper constellation size with a matching symbolrate for available bandwidth and average power constraint, such thateach channel determines the optimal setting for constellation size andsymbol rate.

SUMMARY OF THE INVENTION

[0013] In one aspect of the present invention a method for optimizingdata rate in a digital communication system is provided. In oneembodiment the method and apparatus disclosed herein establishes arelationship between one or more parameters of system operation. In oneembodiment these parameters comprise signal to noise ratio and pluralsymbol rates for a particular constellation size. Thus, in oneembodiment the system may enter a training phase during transmissionoccurs utilizing permutations of one or more symbol rates and one ormore sizes for the constellation or mappings scheme. During this phaseone or more system parameters are monitored and/or recorded, such as thesignal to noise ratio (SNR) and the overall bit transmission rate. Basedon the results of the training phase and optimal symbol rate andconstellation size are determined. These values may fall within desiredtransmit power levels. Also contemplated are modifications to the typeof encoding scheme that is utilized, the modulation scheme, or theequalization strategy.

[0014] As advantages over prior art, the method and apparatus disclosedherein allows an optimal symbol rate and optimal constellation size tobe determined and utilized during a communication session. Othersettings may also be optimized through the training process. Theseoptimal settings may be selected to maximize the signal to noise ratio,minimize signal transmit power, maximize bit transfer rate, reduce echoor crosstalk, minimize bit error rate, simplify implementationcomplexity, or any other aspect of system operation. Through use ofthese optimal settings, communication system operation is improved orenabled.

[0015] Another advantage is that the symbol rate and optimalconstellation size are calculated for the actual installedcommunication(s) devices utilizing the actual channel over which datawill be transmitted. By establishment of the communication systemoperational parameters using the actual installed system over the actualchannel to be used, the most accurate operational parameters may beestablished. Prior art systems select and fix the symbol rate orconstellation size at the time of design suffer from being limited to apredetermined particular setting when other settings may be preferable.

[0016] Yet another advantage of the method and apparatus describedherein is it in one embodiment the constellation size and symbol ratemay be set by with guidance from a technician or to follow certainguidelines. This allows other than the optimal settings to be utilizedbut with emphasis with other parameters. Hence it is contemplated that atechnician may control or limit one or more operational parameters ofthe communication device to achieve desired operation. For example, thefrequency of operation may be reduced, to prevent or reduce crosstalkyet all other parameters, such as transmit power and constellation sizeoptimized. It is contemplated that any variable discussed herein may becontrolled while one or more other parameters are optimized.

[0017] In one embodiment the method disclosed herein may also comprisedetermining noise power spectral density (N (f)), wherein N(f) isdetermined during a silent period of line probing; determining a factorX_(k)(f) based on a nominal front end transmit signal power spectraldensity for a k^(th) symbol rate and an ideal loop gain function of achannel after N(f) has been measured, and determining residual echoE_(k)(f) for a k^(th) symbol rate, wherein E_(k)(f) is determined byactivating a central (local) station echo canceller.

[0018] In another aspect, a method for determining noise power spectraldensity (N(f)) and X_(k)(f) for optimizing bit rate throughput in adigital communication system is provided. The method includesde-activating a remote signal; and measuring N(f). The method alsoincludes activating the remote transmit signal; and measuring X_(k)(f).

[0019] In yet another aspect, a method for determining residual echoE_(k)(f) for optimizing bit rate throughput in a digital communicationsystem is provided. The method includes, activating a local transmitsignal; and activating a local echo canceller.

[0020] In another aspect of the present invention, a system foroptimizing data rate in a digital communication system is provided. Thesystem includes, a spectral analyzer, wherein the spectral analyzermeasures noise power spectral density during a silent period of lineprobing; and an echo channel for measuring residual echo E_(k)(f) for ak^(th) symbol rate. The spectra analyzer also measures X_(k)(f).

[0021] In one aspect of the present invention, a maximum bit rate isachieved utilizing an optimal symbol rate and constellation size withoutsignificant penalties in power.

[0022] In one embodiment, the process and apparatus disclosed herein isoptimized to operate efficiently for both V.90/V.92 and G.shdslstandards.

[0023] This brief summary has been provided so that the nature of theinvention may be understood quickly. A more complete understanding ofthe invention can be obtained by reference to the following detaileddescription of the preferred embodiments thereof, in connection with theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The foregoing features and other features of the presentinvention will now be described with reference to the drawings of apreferred embodiment. In the drawings, the same components have the samereference numerals. The illustrated embodiment is intended toillustrate, but not to limit the invention. The drawings include thefollowing figures:

[0025]FIG. 1 shows a block diagram of a communication system using theadaptive aspects of the present invention;

[0026]FIG. 2 shows a block diagram of a simplified block diagram withvarious components of FIG. 1 for using the adaptive aspects of thepresent invention;

[0027]FIG. 3 shows a flow diagram of process steps for data ratemaximization, according to one aspect of the present invention;

[0028]FIG. 4 shows a flow diagram of process steps for data ratemaximization, according to one aspect of the present invention;

[0029]FIG. 5 shows a simplified block diagram with a spectral analyzer,used for measuring plural parameters for data rate optimization,according to one aspect of the present invention;

[0030]FIG. 6 shows a simplified block diagram with a spectral analyzerfor measuring echo, according to one aspect of the present invention;

[0031]FIG. 7 shows a flow diagram of process steps for measuring echo,according to one aspect of the present invention;

[0032]FIG. 8 shows a block diagram of a power spectral analyzer usedaccording to one aspect of the present invention;

[0033]FIG. 9 shows a flow diagram for the power spectral analyzer usedaccording to one aspect of the present invention;

[0034]FIG. 10 shows a flow diagram for determining maximum data rate,according to one aspect of the present invention;

[0035]FIG. 11 shows an example of a timing diagram for a pre-activationsequence, according to one aspect of the present invention; and

[0036]FIGS. 12A-12C graphically illustrate case studies using theadaptive aspects of the present invention.

[0037]FIG. 13 shows an exemplary block diagram of an example embodimentof the invention.

[0038]FIG. 14 illustrates an operational flow diagram of an examplemethod of operation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0039] Definitions: The following definitions are used in variousaspects of the present invention and with respect to digitalcommunication systems (but not exclusively):

[0040] “Δf_(sym)”: Symbol rate increment;

[0041] “f_(sym,k)”: Symbol rate for the k^(th) allowable symbol rate;

[0042] “PSD”: Power spectral density;

[0043] “BER”: Bit Error Rate;

[0044] “DFE”: Decision Feedback Equalizer;

[0045] “E_(k)(f)”: Residual echo power spectral density for the k^(th)symbol rate;

[0046] “f_(sym-max)”: Maximum symbol rate;

[0047] “FFE”: Fractionally Spaced Feed Forward Equalizer;

[0048] “H(f)”: Ideal loop gain function;

[0049] “M”: Constellation size;

[0050] “MSE DFE”: Mean Square Error Decision Feedback Equalizer;

[0051] “PAM”: Pulse Amplitude Modulation;

[0052] “N(f)”: Noise power spectral density;

[0053] “R”: Data rate ((No of bits/symbol)*symbol rate);

[0054] “STU-C”: Transceiver unit at central office;

[0055] “STU-R”: Transceiver unit at remote office;

[0056] “S_(k)(f)”: Nominal front end transmit signal PSD for the k^(th)symbol rate;

[0057] “SNR”: Signal to Noise Ratio;

[0058] “SNR_(dB)(k)”: Signal to Noise ratio function for the k^(th)symbol rate; and

[0059] “TC-PAM”: Trellis Coded Pulse Amplitude Modulation.

[0060] In general, a method and apparatus is disclosed herein forestablishing optimal communication system settings. To facilitate anunderstanding of the preferred embodiment, the general architecture andoperation of a communication system will be described first. Thespecific process under the preferred embodiment will then be describedwith reference to the general architecture.

[0061]FIG. 13 illustrates a block diagram of an example embodiment of acommunication system configured in accordance with the teachingscontained herein. The communication system may comprise any typecommunication system, including but not limited to, transmission systemsover unshielded twisted-pair lines such as voice-band modems (V.90/V.92modems), DSL transceivers (G.shdsl modems) etc. As shown, a firststation 1330 is configured for communication with a second station 1334.The first station 1330 and second station 1334 communicate via one ormore channels 1312. The first transceiver 1330 connects to the channel212 via an interface 244. The interface 244 is configured to isolate theincoming and outgoing signals.

[0062] The channel 1312 may comprise one conductor or path or more thanone conductor or path, and hence the interface 1344 may performisolation for each channel based on direction of data flow. The channelmay comprise any medium or path. The receive module 1338 and transmitmodule 1342 may comprise any assembly of hardware, software, or bothconfigured to operate in accordance with the principles describedherein.

[0063] The receive module 1338 and transmit module 1342 communicate witha processor 1346. The processor 1346 may include or communicate with amemory 1350. The processor operates as described below in more detailand as would be understood by one of ordinary skill in the art. Thememory 1350 may comprise but is not limited to, one or more of thefollowing types of memory: RAM, ROM, hard disk drive, flash memory, orEPROM. The processor 1346 may be configured to perform one or morecalculations or signal analysis as describe above or as would becontemplated by one of ordinary skill in the art. In one embodiment, theprocessor 1346 is configured to execute machine readable code stored onthe memory 1350. The processor 1346 may perform additional signalprocessing tasks as described herein. For example, the processor maystore a variety of data regarding signal to noise ratio (SNR) or othersystem parameters in the memory. The SNR or other system parameters maycorrespond to different symbol rates or constellation sizes for a givenmaximum transmit power or other session limitations. The term sessionlimitation is defined to mean an aspect of system operation that must befollowed or that system operation is constrained by.

[0064] The second transceiver 1334 is configured similarly to the firsttransceiver 1330. The second transceiver 1334 comprises an interface1352 connected to a receiver module 1356 and a transmitter module 1360.The receiver module 1356 and a transmitter module 1360 communicate witha processor 1364, which in turn connects to a memory 1368. Operationoccurs as described herein.

[0065]FIG. 14 illustrates an operational flow diagram of an exemplarygeneral method of operation. Various methods of operation arecontemplated and that shown in FIG. 14 is but one possible exampleembodiment. Other methods of operation discussed herein provide moredetailed description than that shown in FIG. 14. In generally, thismethod utilizes a training operation to determine an optimal symbol ratesetting and constellation size setting for a p articular communicationsystem while complying with one or more communication system limitationsor session limitations. It is contemplated that this training may occurduring the design and engineering phase, during the install phase at thetime of first use, at the initiation of each communication session, orperiodically during operation, such as for example during idle periods.

[0066] It is contemplated that by utilizing the invention describedherein, the bit rate may be interdependent on the symbol rate, theconstellation size, and possibly one or more other parameters, all ofwhich may be modified

[0067] Turning now to FIG. 14, at a step 1404, a communication systementers a training phase. In one embodiment the training phase occur at atime other than during active communication of user data, however, it iscontemplated that the training phase may occur during activecommunication of user data by simply transmitting user data to insteadof test or training data.

[0068] At a step 1408, the communication system or training module,which may comprise hardware or software, or a combination of both, isprogrammed with the maximum transmit power. This may be fixed bygovernmental regulation, system or channel limitations, or may be inputby a user or technician. As can be understood, a system may havetransmit power limitations due to regional regulatory restrictions,hardware limitations or to reduce crosstalk and echo. At a step 1412,additional transmit parameters or session limitations may be establishedor read from memory. A session limitation comprises any limitation uponcommunication system operation that should be complied with.

[0069] Next, at a step 1416, an initial symbol rate and constellationsize are established. This may be considered the starting point fromwhich training occurs. Any initial symbol rate and constellation sizemay be selected. At a step 1420 the training operation beginstransmitting training data or any other type of data as part of thetraining operation. It is contemplated that this data is transmitted andmonitored to determine optimal settings for the communication systems.In contrast to prior art systems that may have utilized training toestablish filter coefficients, the method and apparatus described hereinestablishes a symbol rate, a constellation size, or both.

[0070] At a step 1424, the system varies the symbol rate and monitorsand records the SNR, or other parameters, for one or more constellationssizes. Thus, in one embodiment the system maintains the constellationsize constant while transmitting data at different symbol rates. Then,at a step 1428, the system may vary the constellation size and monitorand record the SNR or other system parameters, for each of one or moresymbol rates.

[0071] Through the operation of steps 1424 and 1428, a set of data isgenerated regarding system operation, such as for example, the SNR, fornumerous combinations of and permutations of symbol rate andconstellation size. It is contemplated that other parameters, such asthe duration of each training session and/or the number of trainingsessions, may be varied. Any symbol rate and constellation size, withinthe capability of the communication system, may be utilized duringtraining to generate the training data.

[0072] At a step 1432 the training operation stops transmitting thetraining data and at step 1436 the training operation calculates theoptimal symbol rate and constellation size given the system parameters,system limitations or session limitations. In one embodiment the optimalsymbol rate and constellation size are the symbol rate and constellationsize that yield the best SNR. In one embodiment the optimal symbol rateand constellation size are the symbol rate and constellation size thatyield the highest bit transmit rate. In one embodiment the optimalsymbol rate and constellation size are the symbol rate and constellationsize that yield the highest bit transmit rate while maintaining thelowest SNR. In one embodiment a processor, controller, DSP, ASIC, orFPGA performs the processing to determine the SNR, or the optimal symbolrate and/or constellation size.

[0073] Next, at a step 1440, the communication system established thedesired or optimal symbol rate and constellation size calculated in step1436 as the symbol rate and constellation size to be used during acommunication session by one or more communication devices. At a step1444, the communication device performs communication utilizing thesymbol rates and the constellation size, or any other operationalparameter established at a step 1440. This may be considered the optimalsettings or the settings may be established based on any parametersdesired.

[0074] It is contemplated that this training operation may occur withineach communication device in a multiple communication device systemsince the channel characteristics may not be identical in bothdirections. It is also contemplated this training operation may be alsoused to determine receiver equalization settings.

[0075] The following discussion and figures provide more detaildiscussion of the various example embodiments. Returning now to FIG. 1,shown is a block diagram of a communication system 100. The system 100could be located at a central office (may also be referred to as“local”) station (or site) or at a remote station (may also be referredto as “customer premise equipment” site (“CPE”)).

[0076] The system 100 has a transmit path that comprises components 101,102, 103, 107 and 108 (as described below) and is referred to as a“transmitter” or a “modulator”. The system 100 also has a receive paththat comprises components 108, 111, 112, 113, 116, 118, 120 and 122 (asdescribed below) and is referred to as a “receiver” or a demodulator.Since the system 100 comprises of a modulator and a demodulator, it iscommonly referred to as a “modem terminal”.

[0077] The system 100 in the transmit path uses a PAM encoder 102 thatreceives input digital bit stream 10A via a scrambler 101. The PAMencoder 102 modulates digital bit stream 101A and generates a signal102A that is sent to a transmission (Tx) filter 103. The signal 102A isspectrally shaped by the Tx filter 103 and then sent to a linedriver/digital to analog (D/A) converter 105 which converts the digitalsignal 106 into an analog signal Tx 107.

[0078] The analog signal 107 is sent to a hybrid module 108 that routesthe transmit signal 107 to a two-wire twisted pair telephone networkwith some leakage (echo) 107A to the receiver path. Useful Tx signal107B is sent to a remote site (not shown). The signal 107C is thereflection (or echo) resulting from impedance mismatch when the Txsignal 107B is sent from the central station to the remote station.

[0079] The hybrid module 108 is commonly used for full-duplex datacommunications on two-wire twisted pair lines. For efficientperformance, high-speed modems suppress the echo to below backgroundnoise level. The echo occurs due to imperfect impedance match in thehybrid module 108. A digital echo canceller 114 cancels the linear echodue to leakage 107A and 107C.

[0080] In the receive path, a signal Rx 109 enters the hybrid module 108that generates the signal 110 and sends the signal 110 to an analog todigital (A/D) converter 111 that sends a digital signal 111A to areceive (Rx) filter 112. The Rx filter 112 removes any outband noise andreshapes the received signal spectrum. Then the echo estimate 125 issubtracted from the output 112A of the Rx filter 112 to generate asignal 115. The signal 115 is then sent to a FFE 116 that sends thesignal 117 to a decimator 118. The signals 111A, 112A, 115 and 117 arenormally over sampled by a factor of 2 times the symbol rate (e.g. at2/T where T is the symbol period). The decimator 118 decimates thesignal 117 to symbol rate (e.g. at 1/T) and generates a signal 118Awhich is further equalized by a DFE 120. The output of a DFE subtractor121 is sent to a decision device (detector) 122, to generate an output123. The decision device 122 may comprise a symbol-by-symbol PAM sliceror a Viterbi decoder for TCM coded PAM signals.

[0081]FIG. 2 shows a top-level block diagram of a simplified channelmodel used in a communication system (e.g. system 100), according to oneaspect of the present invention. A signal (d(i) where i is the symbolindex) 101B is processed by a transmit segment 201 (described above withrespect to FIG. 1) for a remote site. The output of Tx 201 is shown ass(i) (with its power spectral density, S(f) 202A) 202, which is sent tochannel 203. Channel 203's impulse response maybe designated as h(t) orH(f) in frequency domain). A channel 203 generates x(t) 204 and the PSDof x(t) 204 is given by |X(f)|²=S(f)|H(f)|².

[0082] Additive noise n(t)(its PSD is designated as N(f) 209A) 209comprises of additive white Gaussian noise and cross-talk from adjacenttwisted pairs is added at 205.

[0083] Residual echo e(i) 210 (its PSD is designated as E(f) 210A) afterdigital echo cancellation is sampled at logic 206. A receiver module 207includes the components in the receive path of a system 100 as discussedabove with respect to FIG. 1.

[0084] Determining R_(opt):

[0085] The following describes the technical and mathematical process ofachieving a maximum bit rate with optimal symbol rate and constellationsize without significant penalties in power, with respect to the blockdiagrams of FIGS. 1-2. The variable R_(opt) represents the optimum bitrate.

[0086] If the maximum symbol rate is denoted as f_(sym-max) (in G.shdslits' value is given by f_(sym-max)=2360/3=786.667 kbaud), symbol rateincrement is denoted by Δf_(sym) (in G.shdsl Δf_(sym)=8/3 kbaud) andf_(sym,k)=kΔf_(sym), k=1, 2, . . . , K where K=└f_(sym-max)/Δf_(sym)┘and f_(sym-max)=f_(sym,K) for simplicity and Δf_(sym) may be furtherdivided into L frequency bins, then given the discrete form of the MSEDFE signals, SNR_(dB)(k) for the k^(th) symbol rate may be given by:${{SNR}_{dB}(k)} = {\frac{1}{kL}{\sum\limits_{i = 1}^{kL}{10{\log_{10}( {1 + \frac{{S_{k}( f_{i} )}{{H( f_{i} )}}^{2}}{{N( f_{i} )} + {E_{k}( f_{i} )}} + \frac{{S_{k}( {f_{{sym},k} - f_{i}} )}{{H( {f_{{sym},k} - f_{i}} )}}^{2}}{{N( {f_{{sym},k} - f_{i}} )} + {E_{k}( {f_{{sym},k} - f_{i}} )}}} )}}}}$

[0087] where S_(k)(f) 202A is the nominal far-end transmit signal powerspectral density for the k^(th) symbol rate, |H(f)|² is the magnitudesquared of the ideal loop gain function (H(f)), N(f) 209A is thecross-talk noise power spectral density, E_(k)(f) 210A is the residualecho power spectral density for the k^(th) symbol rate and f_(sym,k) isthe transmit symbol rate. It is noteworthy that a receiver is assumed tohave a 2/T FFE front-end in the above SNR_(dB)(k) formula.

[0088] For un-coded M-ary PAM, exemplary minimum SNRs values forBER<1e−7 are provided in Table 1 below, where M is the constellationsize: TABLE 1 SNRs for BER = 1e-7 M 2 4 8 16 32 64 128 256 512 1024SNR(dB) 11.3 18.4 24.7 30.8 36.8 42.9 48.9 54.9 60.9 67.0

[0089] For TCM-PAM systems, the SNR for a particular M is reduced by theapplicable coding gain. From Table 1 it can be inferred that eachadditional bit/symbol increase requires about 6 dB additional SNR. If Mis reduced to any integer number, then additional SNR for constellationsize increases from M to M+1, which is less than 6 dB and fractionalbits per symbol.

[0090] It is noteworthy that fractional bit per symbols can be achievedby any frame-based mapping methods such as multiple-modulus conversion(MMC), or shell mapping. The higher the bit/symbol resolution, thelarger the frame size and longer the latency.

[0091] Based on the MSE-DFE SNR relationship given above,|X_(k)(f)|²=S_(k)(f)|H(f)|², N(f) and E_(k)(f) can be accuratelyestimated during channel line probing/ranging session, and arelationship between SNRs and various symbol rates f_(sym,k) may beconstructed. This data may be stored in a memory and/or processed todetermine optimal settings. From the SNR(k)−f_(sym,k) relationship andthe required SNR for a particular M size (illustrated in Table 1 as anexample), an optimum achievable MSE-DFE SNR may be ascertained thatachieves BER<1e−7. Then the maximum achievable bit rate for a symbolrate, e.g., R_(k)=f_(sym,k)×log₂(M_(k)) kbps can be determined, andthereafter the optimum bit rate, designated as R_(opt) may bedetermined, as described below with respect to FIGS. 14, 3 and 4.

[0092]FIG. 3 shows a flow diagram for determining R_(k), according toone aspect of the present invention. Turning in detail to FIG. 3, theprocess starts in step S300.

[0093] In step S301, the process starts for the first symbol rate, i.e.,k=1. The k value may be set to any value, but it is contemplated that kis varied during the process of determining optimum values.

[0094] In step S302, the modem terminal is configured to the kth symbolrate. As is well known in the art, this step requires that the clocksignal for the modem terminal be configured to the appropriate symbolrate.

[0095] In step S303, the process estimates N(f) 209A, as described belowin detail. N(f) represents noise power spectral density, which may alsobe considered to be noise.

[0096] In step S304, the process estimates |X_(k)(f)|²=S_(k)(f)|H(f)|².

[0097] In step S305, the process estimates E_(k)(f), as described below.E_(k)(f) represents residual echo power spectral density which may alsobe considered echo.

[0098] In step S306, the process determines the SNR value for the kthsymbol rate. In one aspect this value is determined by using Equation Idescribed above.

[0099] In step S307, the process moves to the next symbol rate, i.e. k+1and in step S308, the process determines if the last symbol rate isreached. If the last symbol rate is not reached, the process moves backto step S302.

[0100] If the last symbol rate is reached, then in step S309, theoptimum rate is determined (described in FIG. 4), and the process stopsin step S310.

[0101]FIG. 4 shows a flow diagram for determining the optimum bit rateR_(opt) as is discussed in step S309 in FIG. 3. Turning in detail toFIG. 4, the process starts in step S400. In step S401, R_(opt) isinitialized to zero and k is set to 1.

[0102] In step S402, the process determines the constellation size (M)for the kth symbol rate. This is determined mathematically as follows:After determining SNR(k) (step S306), the effective SNR γ_(eff)(k) isdetermined by:

γ_(eff)(k)=SNR(k)−Γ+γ_(c)

[0103] where Γ is the predefined noise margin and γ_(c) is theapplicable coding gain if TCM is employed. The bit error rate for M-aryPAM is described in Digital Communications, John Proakis, 3^(rd)Edition, Published by McGraw-Hill, 1995, incorporated herein byreference in its entirety, and is determined by:${P_{PAM}( {{\gamma_{eff}(k)},M_{k}} )} = {\frac{2( {M_{k} - 1} )}{M_{k}}{{Q( \sqrt{\frac{6 \times {\gamma_{eff}(k)}}{( {M_{k}^{2} - 1} )}} )}.}}$

[0104] After determining γ_(eff)(k), the following provides the valuefor M_(k)${P_{PAM}( {{\gamma_{eff}(k)},M_{k}} )} = {{\frac{2( {M_{k} - 1} )}{M_{k}}{Q( \sqrt{\frac{6 \times {\gamma_{eff}(k)}}{( {M_{k}^{2} - 1} )}} )}} = 10^{- 7}}$

[0105] In step S403, the process converts M_(k) to the bit rate for thekth symbol, designated as R_(k). The bit rate for the k^(th) symbol rateis given by:

R _(k) =f _(sym,k)×log₂(M _(k)).

[0106] In step S404 the value for R_(opt) is compared to R_(k). IfR_(opt)<R_(k) is not true then R_(k) is discarded and the process movesto step S406, where the value of k is incremented by 1 (for the nextsymbol rate) and the process moves to step S402. If R_(opt)<R_(k) istrue, then in step S405, the process replaces the potential candidateR_(opt) with R_(k) and saves f_(sym,opt)=f_(sym,k) and the process movesto Step S406.

[0107] In step S407, the process determines if the last symbol rate hasbeen reached. If the last symbol rate has been reached then the processstops at step S408, otherwise, the process steps S402 through S407 arerepeated.

[0108] The foregoing process steps of FIGS. 3 and 4 may be repeated forall possible symbol rates i.e., f_(sym,k), k=1, 2, . . . , K todetermine the optimal symbol rate with matching constellation size thatachieves the maximum bit rate for a given average power limit andmaximum bandwidth. In other embodiments other optimal communicationdevice settings may be calculated with one or more other settings heldconstant or constrained.

[0109] Estimating N(f) and |X_(k)(f)|²:

[0110]FIG. 5 shows a block diagram of a remote transmitter 400 (similarto the transmitter shown in FIG. 1) and a central site receiver 401(also shown in FIG. 1), for determining N(f) 209A and |X_(k)(f)|² whichis equal to |S(f)|H(f)|². Spectral shaper 403 includes a Tx filter 103,and a line driver/D/A converter 105 (FIG. 1), while the receiver 401includes Rx filter 112 and A/D converter 111.

[0111] To measure N(f) 209A, Tx from a remote site may be turned off (orde-activated) such that S(f) 202A becomes 0 and then N(f) 209A isdetermined. |X_(k)(f)|² is determined by turning on (or activating)remote Tx and then measured by spectral analyzer 405. This may occur aspart of the training process.

[0112] It is noteworthy that although the foregoing has been illustratedby activating signals at the remote/central sites, the invention is notlimited to the sites itself. For example, either the remote or centralsite may be used to measure the foregoing values.

[0113]FIG. 6 shows a block diagram of a system for determining E_(k)(f),according to one aspect of the present invention. FIG. 6 shows an echochannel 114B that is used to determine the echo resulting from thesignals 107A and 107C. The echo channel includes various components thathave been shown in FIG. 1, for example, the modules 103, 105, 108, 111and 112. The process steps used by the echo channel 114B will now bedescribed with respect to FIG. 7. The process of echo cancellation isknown in the art and hence the system of FIG. 6 is not discussed ingreat detail. Through monitoring and calculation of the echo on theline, optimal communication device settings may be determined, asdiscussed herein.

[0114] The process starts in step S700. In step S701, the central (orlocal) station (“CO”) transmit side is activated.

[0115] In step S702, echo-canceller (“EC”) 114 is activated and in stepS703 the process determines if EC 114 is converged.

[0116] If the EC 114 has not been converged then the process waits untilEC 114 is converged. If the EC 114 is converged, then in step S704, thespectral analyzer 405 is activated.

[0117] In step S705, E_(k)(f)+N(f) is measured and the processdetermines in step S706 if all the symbol rates are completed. If theyare completed, then in step S707, the process stops, otherwise, theprocess goes back to step S705.

[0118]FIG. 8 shows an example of a block diagram of a spectral analyzer405. The spectral analyzer 405 includes a discrete-time band-pass filter405A and a power meter 405B. The filter 405A receives the output from areceiver and provides a fixed bandwidth and adjustable center frequencyto analyze the signal. Although the spectral analyzer 405 can beimplemented with a discrete Fourier transform (DFT) front-end followedby a power meter, an alternative implementation may use a FIR filtersuch as a FFE filter engine 116 (FIG. 1).

[0119]FIG. 9 shows a flow diagram of FFE implementation used by thespectral analyzer 405. Turning in detail to FIG. 9, in step S900 theprocess starts and in step S901, the frequency bin index value is set tozero.

[0120] In step S902, band-pass filter coefficients are loaded, forexample, one at a time for the ith frequency bin.

[0121] In step S903, the output of the FFE is squared and then averagedto obtain an average power reading in step S904. The results are savedin step S904.

[0122] In step S906, if l is not less than kL (the number of bins to bemeasured for the kth symbol rate), then the process stops in step S907.If l is less than kL, then the process is repeated from step S902, byincreasing the value of l to l+1.

[0123] Applications to G.shdsl (G.991.2) Standard:

[0124] In practice, it is not efficient to estimate the foregoingchannel parameters (i.e., |X_(k)(f)|² and E_(k)(f)|) for every possiblesymbol rate. In fact,${{{X_{k}(f)}}^{2} = \frac{{S_{k}(f)}{{X_{K}(f)}}^{2}}{S_{K}(f)}},$

${E_{k}(f)} = \frac{{S_{k}(f)}{E_{K}(f)}}{S_{K}(f)}$

[0125] and S_(k)(f) are well defined in the G.991.2 recommendation.Therefore, in one aspect of the present invention, |X_(k)(f)|², E_(k)(f)and N(f) are estimated only for the highest symbol rate (i.e., k=K) andSNR(k) is determined as follows:${{SNR}_{dB}(k)} = {\frac{1}{kL}{\sum\limits_{i = 1}^{kL}{10{\log_{10}\begin{pmatrix}{1 + \frac{{S_{k}( f_{i} )}{{X_{K}( f_{i} )}}^{2}}{{{S_{K}( f_{i} )}{N( f_{i} )}} + {{S_{k}( f_{i} )}{E_{K}( f_{i} )}}} +} \\\frac{{S_{k}( {f_{{sym},k} - f_{i}} )}{{X_{K}( {f_{{sym},k} - f_{i}} )}}^{2}}{{{S_{K}( {f_{{sym},k} - f_{i}} )}{N( {f_{{sym},k} - f_{i}} )}} + {{S_{k}( {f_{{sym},k} - f_{i}} )}{E_{K}( {f_{{sym},k} - f_{i}} )}}}\end{pmatrix}}}}}$

[0126]FIG. 10 shows a flow diagram for data rate maximization thatminimizes the amount of line probing in a communication system,according to one aspect of the present invention. Turning in detail toFIG. 10, the process starts in step S1000.

[0127] In step S1001, the modem (system 100 of FIG. 1) is configured formaximum symbol rate.

[0128] In step S1002, N(f) 209A is estimated for the maximum symbolrate. In step S1003, |X_(K)(f)|² is estimated for the maximum symbolrate f_(sym,K)=f_(sym-max).

[0129] In step S1004, the value of E_(K)(f) 210A is estimated for themaximum symbol rate. In step 1005, the value of k is set to one.

[0130] In step S1006, SNR_(dB)(k) is determined, as described above.This value is computed until the last symbol rate is reached in stepS1007. Thereafter, in step S1008, R_(opt) is determined, as described inFIG. 4.

[0131] In step S1009, the process stops. It is contemplated that methodsother than those described in FIG. 10 may be enabled without departingfrom the scope of the claims that follow.

[0132]FIG. 11 shows a timing diagram for using the foregoing processsteps during line probing. G.shdsl allows line probing before data istransferred between a remote and locate site. This process is alsocalled pre-activation. The standard allows a “handshake” for exchange ofparameters between a central and remote site.

[0133]FIG. 11 shows a central site line probing as Pc1 1103 and Pc2 1104with time period Tpcd. During Pc1 1103, Tx signal 107 from central siteis deactivated.

[0134] Line probing Pr1 1101 and Pr2 11102 with time period Tprd are atthe remote site. During time period 1101, Tx signal from a remote siteis deactivated.

[0135] During the silent period 1103, N(f) 209A can be determined, asdiscussed above.

[0136] Case Studies:

[0137]FIGS. 12A-12C show three case studies using the foregoing processsteps, according to the adaptive aspects of the present invention.

[0138]FIG. 12A shows that the optimum achievable rate is 1675 kbps (5bits/symbol) and maximum achievable rate with 16-TCM-PAM (3 bits/symbol)is 1584 kbps (5.7% improvement).

[0139]FIG. 12B shows that the optimum rate is achieved by f_(sym,k)=786kHz, M=5 (or 2.3219 bits/symbol) compared to f_(sym,k)=380 kHz, M=8. Forthis particular case, a combination of lower constellation and widerbandwidth is required to achieve higher rates. The rate difference is1.825 Mbps v.s 1.035 Mbps (76% improvement). This demonstrates thatfixing either symbol rate or constellation is not optimum in terms oftotal achievable data rate.

[0140]FIG. 12C shows that the optimum achievable rate is slightly higherthan that of 16-TCM-PAM. 16-TCM-PAM is near-optimum for most DSL loopscontaminated by AWGN. But it is not optimum when frequency-dependentcrosstalk (NEXT and FEXT) are predominant noise sources.

[0141] It is noteworthy that the foregoing case studies have been usedto illustrate the various adaptive aspects of the present invention andare not intended to limit the scope of the invention.

[0142] In one aspect of the present invention, a maximum bit rate isachieved with optimal symbol rate and constellation size withoutsignificant penalties in power.

[0143] In another aspect, the process of the given system operatesefficiently for both v.90/V.92 and G.shdsl standards.

[0144] While the present invention is described above with respect towhat is currently considered its preferred embodiments, it is to beunderstood that the invention is not limited to that described above. Tothe contrary, the invention is intended to cover various modificationsand equivalent arrangements within the spirit and scope of the appendedclaims.

What is claimed:
 1. A method for optimizing data rate in a communicationsystem, comprising: establishing a symbol rate and a constellation sizefor the communication system; transmitting data utilizing the symbolrate and the constellation size; establishing a first relationshipbetween signal to noise ratio and the first symbol rates and the firstconstellation size; modifying one or more of the symbol rate and theconstellation size to create a modified version of the symbol rate andthe constellation size; transmitting data utilizing the modified versionof the symbol rate and the constellation size; establishing a secondrelationship between signal to noise ratio and the modified version ofsymbol rate and the constellation size; and comparing the firstrelationship to the second relationship to determine an optimal datarate.
 2. The method of claim 1, further comprising determining noisepower spectral density (N(f)).
 3. The method of claim 2, wherein N(f) isdetermined during a silent period of line probing.
 4. The method ofclaim 2, further comprising determining a factor X_(k)(f) based on anominal front end transmit signal power spectral density for a kthsymbol rate and an ideal loop gain function of a channel.
 5. The methodof claim 4, wherein X_(k)(f) is determined by activating a remotetransmit signal, after N(f) has been measured.
 6. The method of claim 1,further comprising determining residual echo E_(k)(f) for a kth symbolrate.
 7. The method of claim 6, wherein E_(t)(f) is determined byactivating a central station echo canceller.
 8. The method of claim 1,wherein the communication system uses digital subscriber lines (DSL). 9.The method of claim 1, further comprising receiving informationregarding signal to noise ratio from a second station, the secondstation receiving the transmitted data.
 10. A method for determiningoptimizing data rate in a digital communication system by monitoringnoise power spectral density, comprising: deactivating a remote transmitsignal; measuring noise power spectral density at a first time;adjusting one or more communication system parameters; measuring noisepower spectral density at a second time; and establishing one or morecommunication system parameters based on the measuring of noise powerspectral density at the first time and the second time.
 11. The methodof claim 10, further comprising activating the remote transmit signaland measuring X_(k)(f).
 12. The method of claim 10, wherein noise powerspectral density is determined during a silent period of line probing.13. The method of claim 10, wherein the one or more communication systemparameters comprise one or more of constellation size and symbol rate.14. A method for determining residual echo E_(k)(f) when optimizing adigital communication system, comprising: establishing an initial symbolrate and an initial constellation size; activating a central stationtransmit signal; activating a central station echo canceller; measuringresidual echo; modifying either or both of the symbol rate and theconstellation size; and measuring residual echo.
 15. The method of claim13, wherein the method for determining residual echo is utilized todetermine an optimal symbol rate and an optimal constellation size. 16.The method of claim 13, further comprising selecting either of theinitial symbol rate or the modified symbol rate or either of the initialconstellation size or the modified constellation size for use during acommunication session.
 17. A system for optimizing data rate in adigital communication system, comprising: a spectral analyzer, whereinthe spectral analyzer measures noise power spectral density during asilent period of line probing; an echo channel for measuring residualecho E_(k)(f) for a kth symbol rate; and a processor configured toestablish an optimal data rate for based on the noise power spectraldensity and residual echo.
 18. The system of claim 17, wherein thespectral analyzer measures X_(k)(f), which is based on a nominal frontend transmit signal power spectral density for a kth symbol rate and anideal loop gain function of a channel after activating a remote transmitsignal.
 19. The system of claim 17, wherein the data rate is controlledby a symbol rate and a constellation size.
 20. A method for maximizingdata rate in a digital communication system, comprising: estimatingnoise power spectral density (N(f)) for a maximum symbol rate;estimating residual echo E_(k)(f) for a maximum symbol rate; anddetermining an optimum data rate (R_(opt)).
 21. The method of claim 15,wherein R_(opt) is determined by: determining a constellation size;converting the constellation size to a bit rate; and comparing R_(opt)with bit rate (R_(k)) for a kth symbol rate.
 22. A system for optimizingcommunication system settings, comprising: a transmitter having one ormore operational parameters, configured to transmit a signal; a receiverconfigured to receive data, from a remote station, regarding the signal;a memory; a processor in communication with the memory, the processorconfigured to: process the signal and the data to determine a firstaspect of operation; modify one or more operational parameters of thetransmitter; responsive to the modifying, process the signal and thedata to determine a second aspect of operation; and determine optimumoperational parameters based on the processing of the signal and thedata with two or more operational parameters.
 23. The system of claim22, wherein the operational parameters comprise symbol rate andconstellation size.
 24. The system of claim 22, wherein the memory isconfigured to store one or more operational parameter settings and oneor more aspects of operation.
 25. The system of claim 22, wherein one ormore aspects of operation comprise aspects of operation selected fromthe group consisting of signal to noise ratio, echo, transmit powerlevel, power spectral density, and crosstalk.
 26. The system of claim22, further comprising performing communication utilizing the optimumoperational parameters.